There are many situations in which electromagnetic energy is to be distributed into an even intensity output requirement. In the vast majority of these situations, a high efficiency transfer of source energy is desirable. This is particularly true in regulated lighting. For example, home and office interior lighting, overland vehicle safety lighting, aircraft lighting, street lamp lighting, and marine lighting are examples that require specific light distribution patterns that are generally mandated by government regulations to have minimum and maximum illumination values. Similarly, corporations have mandated minimum illumination requirements for particular work surfaces. In both cases, a minimum photometric or radiometric output must be met by the illumination device. In many cases, the output distribution requirement consists of an even intensity in angled space or an even illumination projected onto a target surface.
For example, an amber P2 rated sidemarker clearance light requires an even minimum intensity of 0.62 Candellas (Cd) for 45 degrees in the horizontal plane and over 20 degrees in an orthogonal vertical plane as measured by a type A goniometer. For mounting purposes it is desirable to meet the requirement by using an even intensity conical distribution with an output measuring at least 45 degrees from the lamp's central axis.
In another example, for reading lamps, kitchen lamps, or room lighting it is often desirable to generate an even illumination for a conical area over angles ranging from 20 degrees to 70 degrees from the central axis of the lamp. In order to achieve a relatively even illumination, the intensity at the outer edge of the cone is generally higher than in the central axis of the cone to correct for the increased distance to a projection surface, which is typically perpendicular to the axis of the lamp.
Light Emitting Diodes (LEDs) are solid state electrical devices with high efficiencies and long lives. LEDs are generally impact resistant, use very little power and often have 100,000 hour life spans. These features make these devices preferable for use in safety lighting. The primary disadvantage of LED light sources however is their cost. If the efficiency of an optical device to distribute light from the LED into the required or regulated pattern is improved, fewer LEDs can be used resulting in more cost accessible interior illumination and safety lighting devices.
Recently, LED manufacturers have turned to surface mountable LED devices that have superior heat removal from the diode junction and higher optical flux per watt. These devices are now being regularly provided with a flat output surface free from the source distorting optics of past LEDs. These devices typically have very wide output distributions with typical viewing angles greater than 100 degrees. The viewing angle is typically defined as the full angular width of the optical distribution where the light output reaches 50% of the intensity measured on the optical axis. LEDs of this type have generally symmetrical outputs around the center or optical axis. Thus, a device having a viewing angle of 10 degrees describes a conical output distribution where 50% of the peak intensity value occurs at 5 degrees from the optical or center axis of the device. A 120 degree viewing angle device, which is a very common wide output angle LED, defines a device which has an output intensity of 50% at an angle of 60 degrees from the optical axis.
The increased availability of high output LEDs with hemispherical output and intensity closely following that of a Lambertian plane emitter has provided a unique opportunity for the development of new optical lens shapes for meeting government requirements. These LEDs output a highly diffused illumination pattern with a very predictable intensity distribution closely following the trigonometric cosine function. However, a Lambertian LED emitter drops to about 70% of its peak on-axis intensity at 45 degrees. As such, to meet even illumination requirements, 30% more energy must be used.
For interior lighting applications in particular, a smooth output distribution with minimal hot spots or artifacts is aesthetically necessary. Multi-faceted fresnel type optics become impractical for this application as inconsistencies in tooling and manufacturing invariably result in artifacts in the light distribution.
Diffusing lenses have been developed to address some the aforementioned drawbacks of conventional lighting systems. Generally, these lenses reflect over half of the light energy back in the direction of the source preventing it from exiting the lamp. Other energy is often absorbed in the devices themselves. The result is a dramatic increase in the energy source requirement needed to meet specific output distributions. Moreover, higher cost, higher power consumption, and greater package heating also can occur. Thus, these conventional diffusing lenses are generally considered highly inefficient.
Other proposed solutions include lenses with minimal curvature or by employing no lenses at all. In each of these options up to 30% more source energy is required to meet minimum brightness levels adding to overall product cost, increased power consumption, and increased package heating.
It is also worth noting that in the case of LED devices, the diode chip which provides the illumination must be kept to a minimum temperature. Higher LED temperature results in reduced product life and can change the output color and intensity of the LED. Thus, there remains a need for a cost-affordable lamp using one or more LEDs to provide a substantially even intensity conical output distribution.